Manufacture of aragonite

ABSTRACT

CALCIUM CARBONATE IN NEEDLE CRYSTAL FORM IS MADE BY DISSOLVING LIME IN AQUEOUS SUCROSE SOLUTION AND TREATING THE SOLUTION WITH CARBON DIOXIDE, THE SUCROSE BEING FREE FROM CRYSTAL POISONS. IF THE SUCROSE IS TO BE RE-USED IT MUST BE TREATED E.G. BY ANION EXCHANGE RESIN TO REMOVE CRYSTAL POISONS.

June 13, l

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Pump Pre-limer 972 M. c. BENNETT Erm. 3,669,620

MANUFACTURE oF ABAGONITE Filed sept. e. 196s Water Wash Flue Gos rtw I 1"" Sweet Wafer Y :lf Qrum Fxlfer z lie' 1 Drier il CoCOa NaOH oro ire Fllfrofe ro ucf `l` |j 'y'gneronf p g--I Woer Temp x Temp Milk of United States Patent O 3,669,620 MANUFACTURE OF ARAGONITE Michael Camm Bennett and Sidney David Gardiner, London, England, assignors to Tate & Lyle Limited, London, England Filed Sept. 8, 1969, Ser. No. 855,902

Int. Cl. C01f 11/18 U.S. Cl. 23--66 7 Claims ABSTRACT F THE DISCLOSURE This invention relates to an improved form of aragonite and to a method of producing it.

Aragonite is a form of calcium carbonate having needleshaped crystals and physical properties which depend to a large extent on the length of the needles. As available at the present time, manufactured aragonite consists` of dense clusters of needles in which the individual needle length is so small that it cannot be seen even under the optical microscope. This product inds application in pharmaceuticals, in dentifrice, as a ller and as a pigment. It is distinguishable from calcite, the normal form of precipitated calcium carbonate, only by its slightly more fluffy appearance, or by sophisticated tests such as X-ray diffraction measurements.

We have now discovered that aragonite can be manufactured in crystal sizes up to needle lengths of 50 microns. In these larger sizes, it manifests'many new utilities and can be used in many iields, for example as a lter aid, in cosmetics, and as a iiller, as a replacement for the mineral powders already employed. The length of the aragonite needle can to some extent be varied to suit any particular application; for example, as a ilter-aid for raw sugar liquors needle lengths in the range -25 microns are preferred, whilst for iillers and other applications a smaller particle size may be required.

The invention consists in a method of making aragonite of needle length exceeding 3 microns, which comprises dissolving calcium hydroxide in a sucrose solution and reacting this solution at elevated temperature with carbon dioxide, when the aragonate is precipitated, Pure sucrose and some partially rened white sucrose liquors can be used for making the lime-sugar solution. However, without special treatment, the sugar solutions can only be used once for precipitating aragonite, because of the accumulation of traces of interfering materials thought to be polybasic organic acids which are formed by decomposition of sucrose during the reaction and which prevent the precipitation of the desired aragonite, yielding calcite instead. Some partially refined sugar liquors, for example liquor known in the industry as ne liquor, may already contain traces of the same organic acids and these liquors require treatment before use.

The required aragonate can be made continuously if a continuous supply of fresh sugar or liquor is available, for example in a sugar factory or refinery. Alternatively, according to an important aspect of the invention, the sugar solution can be recycled provided the inuence of the interfering crystal poisons is removed. There are two procedures of which we are aware by which this can be achieved. One is to remove the organic acids altogether by the use of an anion exchange resin, or a suitable absorbent, e.g. a carbonaceous absorbent, the other is to remove them from the precipitation reaction by rendering f a 3,669,620 ce Patented June 13, 1972 them unavailable, e.g. by progressively adding alkali to the recycled sugar solution. The amount of alkali, such as caustic soda, which is required may be determined in any individual case by simple trial, it will lie within the range 0.1 to 3.0 milliequivalents per litre of NaOH; inevitably, of course, this will lead to an excessive accumulation of sodium ions.

It is preferred that the sucrose concentration should be between 20 and 50 Bx. It is preferred that the carbonation of the lime-sugar solution should be conducted at an elevated temperature of for example 60 to 90 C. at a pH exceeding 7, suitably between 7 and 9 measured at the temperature of the precipitation. It is preferred that the lime concentration in the sugar solution should be between one-half and one-twentieth molar.

The aragonite crystal size will, of course, depend to some extent upon the residence time in the lime-sugarcarbon dioxide reaction mixture. It is preferred that the residence time should be between 15 minutes and 1% hours. Mainly, however, the aspect ratio is determined by the temperature of the liquor at the liming stage. Thus lwhen liming is conducted at C. the crystal width is about ll/z microns and the mean length 35 microns whilst at 65 C., the mean width is about 21/2 microns and the mean length about 20 microns. The aragonite needles are usually precipitated in clusters up to 200 microns in diameter, although separate needles may be formed if desired, by breaking upl the clusters.

The process of this invention is illustrated in the accompanying flow sheet which indicates the sequence of operations required for the continuous recycling of the same sugar solution. The flow sheet shows two ion-exchange resin cisterns through which the sugar solution is passed after separation of the precipitated aragonite: these two cisterns are used alternately to permit regeneration of the resin without interrupting the continuous carbonation reaction.

In a typical process according to the invention, a reactor is used having a residence volume of 100 l. and the sucrose solution containing dissolved lime is fed therethrough at a temperature of C. at a rate giving a residence time of l hour. A ue gas is fed through at a rate of 170 litres per minute at 40% O02 absorption, the liquor ow rate being litres per hour. The liquor used is a white liquor diluted to 40 Bx., the liquor entering at 39 Bx. and leaving at 41 Bx. because of some degree of evaporation in the reactor. The lime is used at a feed rate of 1.85 kg. per hour to give a one-quarter molar concentration. The resin which is employed is that known as IRA 400 in the hydroxyl form and the sucrose solution is treated at a maximum temperature of 60 C. The reactants described give aragonite at a rate of 2.5 kg. per hour having a -settled bulk volume of 15-30 cc. per g. The mean crystal size is:needle length of l5 microns, mean diameter 3 microns. If the treatment with ionexchange resin is omitted, the material obtained from the drum iilter is calcite instead of aragonite,

The liming of the sucrose solution may be conducted over a wide range of temperature, e.g. 50-90" C., the higher temperatures giving the longer, thinner needles.

We claim:

i1. A method of making aragonite which comprises dissolving calcium hydroxide in an aqueous solution of sucrose, reacting this solution at from 60 to 90 C. with carbon dioxide in the absence of crystal poisons, said crystal poisons comprising organic acids which cause precipitation of calcite instead of aragonite, and separating the precipitated aragonite, the sucrose concentration in said solution being between 20 and 50 Brix, the carbonation of the calcium hydroxide-sucrose solution being conducted at a pH of between 7 and 9 measured at said reaction temperature of from 60 to 90 C., the

calcium hydroxide concentration in the sucrose solution being between one-half and one-twentieth molar, and the residence time 0f the precipitated aragonite in the calcium hydroxide-sucrose-carbon dioxide reaction mixture being between 15 minutes and 111/2 hours.

2. A method according to claim 1, wherein said sucrose solution contains said crystal poisons, further comprising the step of rendering said crystal poisons unavailable prior to reacting with carbon dioxide.

3. A method according to clairn 2, wherein said crystal poisons are rendered unavailable by treatment with an anion exchange resin.

4. A method according to claim 2, wherein said crystal poisons are rendered unavailable an by the addition of an alkali.

5. A method according to claim 1 in which the sugar solution left after separation of the precipitated aragonite is treated to render the crystal poisons unavailable and re-used as said aqueous solution.

References Cited UNITED STATES PATENTS 2,188,663 1/ 1940 McClure et al. 23--66 2,269,608 v 1/1942 Strieby v 23-66 X 3,340,003 9/1967 Judd 23-66 `3,443,890 5/19'6-9 SiSsOn et al. 23--66 OSCAR'R. VERTIZ, Primary Examiner G. ALVARO, Assistant Examiner 

